Description:
The patch gene is a mutation in pigs on the white (KIT) locus (1)(2). Unlike dominant white or belt, it creates white patches through the stomach and face with sharply defined borders(1). It was officially mapped in 1992 in Swedish laboratories (4). When combined with the red gene, it creates the look seen in Hereford pigs and some crossbreds. In general, it is very similar to mutations in mice and humans seen on chromosome 5 and 4 respectively.
It is an incomplete dominant mutation (4). Pigs with one copy of the gene will typically have less white and smaller patches than pigs with two copies of the gene.
The gene symbol is I^P, and the order of dominance on the locus is No White < Patch < Belted < Dominant White, or i+<IP<IB<ID.
Breeding with Patch:
Patch x Patch = 100% Patched pigs
(P/P x P/P = 100% P/P)
Patch x White = 100% White pigs that carry the Patch gene
(P/P x W/W = 100% W/P)
Patch x Belted = 100% Belted pigs with white patterning through the face and belly
(P/P x B/B = 100% B/P)
Patch x All Red = 100% Het. Patch (smaller white spots)
(P/P x i+/i+ = 100% P/i+)
Patch x Het. Patch = 50% Patch, 50% het. Patch
(P/P x P/i+ = 50% P/P, 50% P/i+)
Het. Patch x Het. Patch = 25% Patch, 50% het. Patch, 25% No Patch
(P/i+ x P/i+ = 25% P/P, 50% P/i+, 25% i+/i+)
Het. Patch x No Patch = 50% Het. Patch, 50% No Patches
(P/i+ x i+/i+ = 50% P/i+, 50% i+/i+)
References & Further Reading
Marklund, Stefan, et al. "Molecular basis for the dominant white phenotype in the domestic pig." Genome research 8.8 (1998): 826-833.
Pielberg, Gerli, et al. "Unexpectedly high allelic diversity at the KIT locus causing dominant white color in the domestic pig." Genetics 160.1 (2002): 305-311.
Moller, M.J., Chaudhary, R., Hellmén, E. et al. Pigs with the dominant white coat color phenotype carry a duplication of the KIT gene encoding the mast/stem cell growth factor receptor. Mammalian Genome 7, 822–830 (1996). https://doi.org/10.1007/s003359900244
Johansson, Maria, et al. "The gene for dominant white color in the pig is closely linked to ALB and PDGFRA on chromosome 8." Genomics 14.4 (1992): 965-969.